An optical fiber (i.e., a glass fiber typically surrounded by one or more coating layers) conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the optical cladding ng (i.e., nc>ng).
For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “parabolic” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, a parabola, or a triangle. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
Generally speaking, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber. In a single-mode optical fiber, the signal propagates in a fundamental LP01 mode that is guided in the optical-fiber core, while the higher order modes (e.g., the LP11 mode) are strongly attenuated.
Amplifying optical fibers (e.g., optical fibers doped with rare-earth elements) are commonly used in various optical applications.
International Publication No. WO 2010/080952 (and its counterpart U.S. Patent Publication No. 2010/0178504), each of which is hereby incorporated by reference in its entirety, disclose bend-insensitive optical fibers with a low refractive index glass ring in the cladding (i.e., a ring where the refractive index percent relative to pure silica glass is less than −0.5 percent). The design of the disclosed optical fibers, however, does not satisfactorily reduce bending losses. Additionally, International Publication No. WO 2010/080952 does not target high-power applications.
Thus, there exists a need for a rare-earth-doped amplifying optical fiber for use in a compact configuration and high power applications (e.g., around 500 mW or more).